The invention relates to the electrical measurement art, and more particularly to a new improved sensor or probe for use as a non-contacting electrostatic field, electrostatic voltage, or electrostatic charge detecting device.
Non-contacting electrostatic detectors are employed in those applications where electrical charge transferred to the measuring device through physical contact with the surface, circuit, or device being measured/monitored would modify or destroy the data. Such measurements include, for example, monitoring the voltage level of a photoconductor used in a copy machine or other electrophotographic processes, monitoring of electrostatic charge build-up on a dielectric web process line such as is used in textile or plastic film manufacturing processes, or monitoring the area around semiconductor manufacturing/handling processes where charge accumulation presents a source of product contamination or destruction through electrostatic discharge (arcing) events.
Heretofore, electrostatic sensors/probes of the type required to produce high accuracy, fast speed, low noise measurements, and yet be of physically small size, have been too expensive to manufacture to allow their use in higher quantity lower cost processes and/or apparatus such as low end laser printers, copy machine, or multiple point electrostatic charge build-up monitoring systems.
It is, therefore, a primary object of this invention to provide a new and improved sensor/probe for electrostatic measurements which satisfies the foregoing requirements yet is of low cost to manufacture.
It is a further object of this invention to provide a sensor/probe having electrical performance independent of both the mass of the sensor/probe and/or the mass of the clamping/holding structure used to place the sensor/probe into its measurement position relative to the surface or area under test.
It is a further object of this invention to provide such a sensor/probe which minimizes or more easily eliminates the production of noise and offset signals thereby avoiding the need for providing relatively large spacing between electrical elements of the sensor/probe amplifier circuitry and sensor/probe housing elements thus reducing the ability of contamination to enter into the inside of the sensor/probe structure.
It is a further object of this invention to provide a sensor/probe which can be easily configured to provide various spatial resolution characteristics.
It is a further object of this invention to provide a sensor/probe having an electrical speed of response which is substantially increased so as to accurately measure more rapidly changing electrostatic parameters.
It is a further object of this invention to provide such a sensor/probe which is small in size, simple in construction, economical to manufacture and reliable in operation.
The present invention provides a sensor for an electrostatic detector comprising an elongated vibratory element supported at a mechanical node at one end in the manner of a cantilever beam, a sensitive electrode on the vibratory element near the other end and adapted to be disposed toward an electrical charge, field or potential being measured, and driver means on the vibratory element for vibrating the element in a direction to vary the capacitive coupling between the electrode and the electrical field or potential being measured. The driver means is at a location along the beam and is operated at a vibratory frequency such that a virtual mechanical node appears along the beam. This, in turn, reduces vibration at the mechanical attachment node so that the degree of stiffness of the mechanical node does not affect the operating frequency and/or the displacement at the free end of the cantilever. The variation of the vibration amplitude of the beam and thus the detector""s motion amplitude at the end of the beam is made independent of both the mass of the sensor/probe and/or the mass of the structure, such as a clamping/holding fixturing, used to place the sensor/probe into its measuring position relative to the surface or area under test. The electrical performance is thus stabilized and is made independent of the particular clamping/holding structure used. An amplifier on the vibratory element has an input connected to the sensitive electrode and an output adapted for connection to a readout/measuring device such as an electrostatic voltmeter. According to a preferred embodiment of the present invention the vibratory frequency corresponds to the second vibrational node of the beam and the driver means is located on the beam in overlapping relation to the peak of the beam vibration near the middle thereof.
The foregoing and additional advantages and characterizing features of the present invention will become clearly apparent upon a reading of the ensuing detailed description together with the included drawing wherein: